Magnesium has a low boiling point compared with most other metals and this is why many processes for the recovery of the raw metal or also for the recycling of magnesium from scrap metal are performed via the process step of vacuum distillation, since a magnesium that is largely purified from less volatile metals can be recovered in this way in a single step. If it is also sought to remove these volatile substances in order to produce high-purity magnesium, for example as is desirable for the semiconductor industry, vacuum distillation facilities having a plurality of condensation regions arranged in series are used, and therefore high-purity fractions, in which the impurities are merely in the ppm range, are also obtained from a number of fractions contaminated to a significant extent by other volatile metals, such as zinc and cadmium. Such a process is described in EP 1 335 030 A1, wherein the steam rising from a crucible containing an impure magnesium melt is conveyed via a multiplicity of successive deposition plates heated to decreasing temperatures and deposits on these plates in fractionated form.
The evaporation temperature of magnesium can be lowered with reduction of pressure until below the temperature of the melting point, and a particular feature of this metal is that its steam pressure even below the melting point is still so high that it is sufficient for a technically useful resublimation of high-purity magnesium crystals. Accordingly, most known vacuum distillation processes in accordance with the prior art for producing high-purity magnesium lead to the deposition of solid magnesium crystals.
Such magnesium crystals, in view of their low content of foreign elements, are indeed referred to as high-purity in the chemical sense, but the crystals have a high surface/volume ratio, and, when such crystals are remelted for the purpose of producing a semifinished product or near-net-shape articles, the oxide skins originally present on the surface of the magnesium crystals due to the high reactivity are distributed as non-metal inclusions in the melt and remain in the solidified material. Although they have only low concentration values, such inclusions, however, can adversely influence the corrosion behaviour of the otherwise high-purity magnesium, for example.
In accordance with EP 1 335 032 A1, there is a process in which an impure magnesium melt is evaporated in an evaporation vessel from high-purity graphite, wherein this steam then precipitates as liquid melt in a condensation crucible likewise consisting of graphite. Both crucibles are surrounded by a bell made of graphite, which prevents the magnesium steam from coming into contact with the cold wall of the vacuum retort surrounding the bell and condensing there. In order to bring both the evaporation crucible and the condensation crucible to the temperatures necessary for the process and simultaneously to keep the retort cold, two heating elements are present in the gap between the retort wall and the graphite bell. In particular due to the mounting of the heating elements within the vacuum region and the protection of the actual evaporation zone and condensation zone by the graphite bell, an increased structural outlay is created and in addition the inner volume has to be evacuated through leaks of the graphite bell, whereby magnesium steam can also pass externally at these points to the heating elements and the cold retort wall.